Abstract: The net lifting force of pilot balloon has been computed by approximation method in this paper. A new table for this force is given based on the computations.The error of the lifting velocity caused by the net lifting force has also been computed.The results indicate that: in the old table which was currently used in the meteorological station, the error of lifting velocity does not follow a normal distribution, its maximum value was as high as 3.22m/min; the new table we suggested, the error is normaly distributed, its maximum and mean values are reduced to 1.36m/min and 0.05m/min respectively.